Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogenfixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining "vicious circle." To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential.
The copepod Acartia tonsa has 2 different prey encounter strateg~es. It can generate a feeding current to encounter and capture immobile prey (suspension feeding) or it can sink slowly and perceive motile prey by means of mechanoreceptors on the antennae (ambush feeding). We hypothesized that A. tonsa adopts the feeding mode that generates the highest energy intake rate; i.e. that prey selection changes according to the relative concentrations of alternative prey (prey switching) and that the copepods spend disproportionately more time In the feeding mode that provides the greatest reward. Based on earlier observations, we also hypothes~zed that turbulence changes food selection towards motile prey. We tested these hypotheses by examining feeding rates and behaviour in adult females of A. tonsa feeding in mixtures of 2 prey organisms, a diatom (Thalassiosira welssflogjj) and a ciliate (Strombidium sulcatum). Our data demonstrate prey switching in A. tonsa, both in terms of behaviour and in terms of feeding rates on the alternat~ve prey. The time allocated to ambush and suspension feeding changed with the composition of the food, and clearance of diatoms was, accordingly, negatively related to the availability of ciliates. In contrast, clearing of ciliates was almost constant and independent of the availability of the alternative prey (diatoms), probably because this particular ciliate species (in contrast to most other microzooplankters) is unable to escape a feedlng current and, thus, can also be captured by suspension feeding copepods. Finally, we demonstrate that turbulence favours the selection of cillates as prey. We suggest that prey switching by copepods may provide survival windows for microzooplankters during blooms of net phytoplankton because predation pressure from the copepods is then less. This may explain why microzooplankton populations often peak concurrently with net phytoplankton blooms and apparently independently of their own food.
We investigated the vulnerability of 2 copepod species (Eurytemora affinis and Temora longicornis) to predation by predators with different foraging modes, three-spined stickleback Gasterosteus aculeatus juveniles and mysid shrimps Neomysis integer. Copepods were videofilmed escaping from predators and from an artificial flow field, and the results were used in a model of hydrodynamic disturbance generated by a predator The copepods detected mysids from a significantly larger distance than they detected sticklebacks (0 45 and 0.24 cm, respectively). Consequently, the capture success of the sticklebacks was higher than that of mysids. In the case of sticklebacks foraging on E. affim-S, copepod reaction distance was significantly correlated with stickleback approaching speed; sticklebacks captured a copepod only if they were able to slowly approach to within a strike distance of <0.1 cm from the prey. Also, there was a major difference between the vulnerabilities of the 2 prey species: the capture success of sticklebacks was 92 % with T. longicornis and 53 % with E. affinis. This corresponded with experiments with artificial flow, where the threshold fluid velocity gradient eliciting an escape response in copepods was 4 times higher in T, lonqicornis than in E, affjnis (8.2 and 2.1 S-', respectively). The hydrodynamic model accurately predicted the positive relationship between stickleback approaching speed and copepod reaction distance, as well as the difference between the 2 copepod species. This suggests that, by using simple artificial flow experiments, we can rank various zooplankton species according to their escape capabilities, and thus predict their vulnerability to predation by small fish with different motility patterns. In contrast, the model did not conform with observations on mysids. Apparently, the hydrodynamic disturbance created by a mysid is not related to its swimming speed, but to some other factor, such as the beat rate of swimming appendages.
During the stagnation period of the Baltic Sea the mean weight-at-age of Baltic herring decreased by 50% (between 1977 and 1992). This has usually been attributed to a top-down process, i.e. to the simultaneous collapse of cod stocks and their predation. We present long-term data for 1980 to 1993 showing that bottom-up effects may also have played a role: along with the decline of salinity. the biomass proportion of zooplankton taxa preferred by herring (larger than 20 pg ind:' in wet welght) significantly declined. To support our hypothesis we present a study in which Baltic herring feeding and selective predation were investigated during 1985, a time of good growth and high weightat-age, and 1991, when herring growth and weight-at-age were poor. In this study, herring ston~achs and simultaneously taken plankton samples were analysed from trawl surveys conducted in the northern Baltic proper d'uring the peak of the herring feeding season in late summer. During both 1985 and 1991, herring selectively preyed on the larger zooplankton categones, especially neritic copepods. However, in 1991, a smaller proportion of the prey in herring stomachs consisted of neritic copepods, apparently because their share in plankton had decreased. Consequently, and despite an increase in total zooplankton biomass, the estimated carbon content of the food eaten by herring was lower, and the average stomach fullness index (on a scale of 0 to 5) decreased from 3.9 in 1985 to 1.9 in 1991. Also, the amount of mesenteric fat on herring stomachs declined from 4.2 to 3.2 (scale 0 to 5), indicating a longer-term failure in feeding success. We suggest that, in addition to possible top-down effects (a release of cod predation), bottom-up processes mediated via changes in mesozooplankton species composition have also influenced hernng growth and that both of these processes are affected by the same environmental factor-the Baltic salinity level.
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